To determine the moisture content in a sample, the sample is dried and the weight of the sample is measured before and after the drying process. Due to the extensive amount of work involved, this method is very expensive as well as error-prone.
In some cases, the weight loss can also be measured during the drying process. In a given sample, the decrease in weight is a function of the temperature, the length of the drying time, and the conditions in the test compartment, and it conforms to a weight-versus-time curve which asymptotically approaches the dry weight of the sample. The curve for the given sample is determined by comparative experiments and can be expressed mathematically through an approximation formula. A measuring instrument for gravimetric moisture determination which is appropriately equipped with available electronic technology can compute the moisture content of a sample based on the measured parameters of the aforementioned curve and based on the length of the drying time and indicate the result on a display unit. With this method, the substance to be dried does no longer need to be totally desiccated; it is sufficient to determine the coordinates of two measurement points in the weight-versus-time diagram.
As has already been mentioned at the beginning, the weight change of a sample is substantially a function of the temperature, the length of the drying time, and the conditions in the test compartment. Especially the stringent requirements imposed on the test compartment and its design features are setting a limit to the accuracy of the commercially available instruments.
The term “test compartment” in the present context means a space which is enclosed by the housing of the measuring instrument and which can be opened in order to insert or remove a sample. Also arranged inside the test compartment are a sample receiver and a means to heat the sample. The sample receiver is connected to a gravimetric measuring instrument.
Normally, the sample is spread in a thin layer onto a flat sample receiver, for example a sample tray. The tray is preferably arranged in the measuring instrument for gravimetric moisture determination in such a way that the sample-carrying area is horizontally leveled, so that samples of low viscosity cannot collect at the lowest point (relative to the direction of the load) of the sample tray.
As a means for heating the sample, a variety of radiation sources are used, such as heat radiators, microwave generators, halogen- and quartz lamps. A gravimetric moisture-determination instrument of the aforementioned type is disclosed in commonly-owned U.S. Pat. No. 5,485,684, issued 23 Jan. 1996 to Philipp, et al. In this instrument, the sample substance is put on the weighing pan while the latter is outside of the gravimetric moisture-determination instrument. To do this, the balance is pulled out of the housing of the measuring instrument on a sliding carrier like a drawer. For a radiation source, a ring-shaped halogen lamp is used which is located above the sample receiver when the instrument is in its operating condition.
As was found in experiments, the type and the design configuration of the radiation source being used are among the primary causes for inaccurate measurement results in existing gravimetric moisture-determination instruments. For example, radiators with perforations or radiators whose radiation originates substantially from a point or a line can cause a non-uniform irradiation of the sample with the result that the energy density in individual spots of the sample can be so high as to cause in some places a thermal breakdown of the sample.
If the radiator spans over the sample in a spread-out and largely flat configuration, it is possible that a moisture-saturated gas cushion will form between the sample and the radiator and remain in place, whereby a further escape of moisture from the sample is prevented. Such an obstruction to the drying process could have a significant effect on the drying time, wherein in particular the temperature-related random atmospheric convection between the radiator and the sample enter into the measuring result.
The errors in the drying time that are caused by the obstruction in the drying process, and/or the measurement errors in the sample weight values due to thermal decomposition impose a limit on the accuracy that can be obtained in an analysis with the aforementioned mathematical model. As an alternative to using the mathematical model, one can use the known method in which all of the moisture—to the extent that this is possible—has to be driven out of the sample. However, this requires a very long drying time, which increases the risk that a thermal decomposition or oxidation of the sample will occur as a result of the long, sustained exposure to the heat radiation of the radiators.
For the reasons that have just been explained, it is hardly possible to determine an absolute value for the moisture content with a gravimetric moisture-determination instrument. For a more accurate determination of the moisture content of a substance or for the calibration of dryers, the known Karl Fischer titration method is therefore still in use. This method is very labor-intensive, prone to user errors, and expensive.
It is therefore an object to provide a gravimetric moisture-determination instrument of the kind mentioned in the introduction with a radiator that has an improved distribution of the radiation over the sample. Furthermore, the escape of moisture from the sample should not be compromised as a result of the improved distribution of the radiation.